1. Field of the Invention
The present invention relates to the protection of spacecraft from hypervelocity impactors and more particularly to a stand-alone, self-contained impact shield assembly for protecting a spacecraft surface from such hypervelocity impactors.
2. Description of the Related Art
Designing spacecraft for the future has introduced many new challenges. One such challenge is to provide affordable, maximum protection against the ever increasing threat of man-made orbital debris. As spacecraft soar into near-earth orbits they share orbital space with thousands of large and perhaps tens of millions of medium-sized debris objects left from 3,800 previous space missions since 1957. It has been estimated that the total amount of debris orbiting below 1,240 miles is between 8.5 and 10 millions pounds and is increasing. The average accidental impact velocity between spacecraft and orbital debris in this region is 10 kilometers per second (22,300 miles per hour) and can reach velocities as high as 15 kilometers per second (34,000 miles per hour). At these velocities, a piece of debris no larger than a garden pea carries the destructive punch to potentially cripple an entire unprotected spacecraft.
Currently, spacecraft (particularly manned spacecraft) use bulky, heavy metallic "bumpers" to protect critical areas. These bumpers, which usually consist of thin aluminum sheets placed 4 to 8 inches off the surface they are protecting, are designed to be perforated when impacted by a meteoroid or debris particle while shocking the particle and breaking it into fragments. These fragments disperse in an expanding debris cloud which spreads the impact energy over a much larger area, making it far less damaging to the surface behind it. If the impacting velocity is high enough, melting or vaporization will occur, which aids in reducing impact lethality.
The shortcomings associated with the standard metallic bumper concept are numerous. First and most importantly, metallic bumpers are very heavy. Since bumpers are the outer-most spacecraft surface, their large surface area translates to excessive weight. In addition, in order to survive the rigors of launch, bulky structure is necessary to support these bumpers. The industry rule-of-thumb is that metallic bumpers and support structure are expected to contribute approximately 5-10% to the total vehicle weight.
Secondly, high performance metallic bumper concepts often violate payload volumetric constraints. The volume consumed by such designs is significant when bumper shields protrude out 4 to 8 inches from the vehicle surface they are protecting. This space is preferable for functional equipment.
Thirdly, metallic bumper shields often experience thermal expansion and heat transfer related problems on-orbit because of the extreme temperatures. Design complexity is often necessary to alleviate these problems. Finally, metallic bumpers offer no real means of on-orbit augmentation without excessive vehicle design and/or configuration impacts.
U.S. Pat. No. 5,067,388, issued to Crews et al. discloses a hypervelocity impact shield that utilizes layers of impactor disrupting material which are spaced apart to progressively shock the incoming meteoroid or orbital debris particle to a vapor state which increases the penetration resistance dramatically. This concept is believed to provide the maximum passive protection of any heretofore existing shield design. The major shortcoming of the '388 shield is that there is no practical or efficient means disclosed in that patent for spaceably supporting the impactor disrupting material.
Present applicant is aware that current applications of the concepts described in the '388 patent have involved conventional rigid metallic support structures which are heavy, bulky and cumbersome. Such support structures prohibit their usefulness for spacecraft applications.
U.S. Pat. No. 5,610,363, also issued to Crews et al. discloses a hypervelocity impact (HVI) whipple shield and a method for shielding a wall from penetration by high velocity particular impacts where the whipple shield is comprised of spaced apart inner and outer sheets or walls with an intermediate cloth barrier arrangement comprised of ceramic cloth and high strength cloth which are interrelated by ballistic formulae.